Traditionally, drilling for oil and gas involved using a drill bit to drill boreholes that were straight and more or less vertical. As their skill and equipment improved, drillers found that they could deviate from a straight path to form a curved borehole. One application of the somewhat limited directional drilling techniques allowed plural boreholes to be drilled from a small location. For example, plural wells could be drilled through a leg of an offshore platform. Although the boreholes were very close to each other at the surface, the bottoms of the boreholes were separated in different locations of an oil reservoir or even in different reservoirs.
Improvements in directional drilling have allowed drillers to drill a borehole in just about whatever direction is required. A borehole can be truly horizontal, where it lies along a horizontal plane. A borehole can even be drilled upwardly towards the surface from a lower location. This type of drilling is referred to as horizontal drilling and has allowed production increases from oil fields once thought diminished or even exhausted.
Now that drilling techniques and equipment can locate a borehole along almost any orientation, the problem of navigating the borehole during drilling arises. Oil is typically located in thin stratigraphic zones. Ideally, the driller would like to tap into the stratigraphic zone of interest, or target zone, with a borehole that traverses inside of the zone for an extended distance. For example, if the target zone has a true horizontal orientation, then the borehole, when it penetrates into the zone, extends along the horizontal to stay within the zone.
Unfortunately, the prior art only provides hit or miss techniques in maintaining a borehole inside of a target zone. The target zone is typically thousands of feet below the surface and is, in many instances, only 5-20 feet thick. Furthermore, stratigraphic zones are typically inclined or dipped from a horizontal plane. Thus, the target zone is a difficult target in which to maintain the borehole during drilling operations. With prior art drilling techniques, it is difficult to navigate a borehole so as to stay within a target zone. A borehole drilled with prior art techniques quickly exits the zone because its direction is not parallel to that of the zone. What is needed is a method of navigating a borehole inside of a stratigraphic zone during drilling.